Geological methods applied to speleogenetical research in
Transcripción
Geological methods applied to speleogenetical research in
Carbonates Evaporites (2011) 26:29–40 DOI 10.1007/s13146-011-0052-7 ORIGINAL ARTICLE Geological methods applied to speleogenetical research in vertical caves: the example of Torca Teyera shaft (Picos de Europa, northern Spain) Daniel Ballesteros • Montserrat Jiménez-Sánchez Joaquı́n Garcı́a-Sansegundo • Santiago Giralt • Accepted: 19 February 2011 / Published online: 5 March 2011 Ó Springer-Verlag 2011 Abstract Research in large vertical caves (shafts) is rare and usually restricted to speleological explorations because of difficult access. The systemic methodology of work in shafts has not been established. Picos de Europa massif, in the Cantabrian Mountains of Spain, has a spectacular development of shafts deeper than 500 m. One of them is Torca Teyera cave, which is 738 m deep and 4 km long. The present study established a methodology to characterize the geological and geomorphological aspects of this special group of caves and to identify the factors contributing to karst development. The research is multidisciplinary, needs data from the cave and the caves’ surroundings and involves (1) the speleological cave survey at a 1:500 scale: the construction of a 3D model and morphometric analyses; (2) the geomorphological mapping on the cave survey at 1:500; (3) the geological and fracture mapping of the cave environment and cross section at 1:5.000; and (4) the comparison in stereographic projection of the obtained survey data and joint measures. Keywords Karst Shafts Speleogenesis Picos de Europa Geomorphological maps Structural control D. Ballesteros (&) M. Jiménez-Sánchez J. Garcı́a-Sansegundo Departamento de Geologı́a, Universidad de Oviedo, C/Arias de Velasco s/n, 33005 Oviedo, Spain e-mail: [email protected] S. Giralt Instituto de las Ciencias de la Tierra Jaume Almera (CSIC), C/Lluı́s Solé i Sabarı́s s/n, 08028 Barcelona, Spain Introduction Research in vertical caves or shafts is typically limited due to difficult access and methodological constraints. Scientific studies of these require speleological exploration, which can often include the discovery of new caves and passages, as well as careful documentation (Kambesis 2007) such as cave surveys, exploration reports, photographs and morphological descriptions of the cavities. The exploration of large shafts in Europe began in the late 1970s with the publication of several speleological studies on the Alps, Slovenia, the Pyrenees and the Picos de Europa Mountains. Since 1981, the Oxford University Cave Club has explored the shaft Pozu del H.itu (1,135 m deep; Singleton and Naylor 1981). The calcareous massif of the Picos de Europa is considered as one of the prime sites for investigation by speleologists owing to the spectacular development of large shafts (e.g., Ogando 2007). The speleological documentation in Picos de Europa is extensive, but it is neither systematized nor inventoried. The main karst systems are well known through speleological publications, which sometimes include geological observations (e.g., Erheyden et al. 2008). Some geological research in large caves of this massif has been developed from speleological explorations (e.g., Laverty and Senior 1981; Senior 1987). Currently, Picos de Europa contains 13% of the shafts known in the world to be deeper than 1,000 m. Most of the karst systems have shafts of only a few kilometers. The deepest shaft in this system is Torca del Cerro del Cuevón, which is 1,589 m (Estévez 1998), and the largest cave system is the Red del Toneyu, with a development of 18,970 m (Gea 1991). Nevertheless, few works have focused on endokarsts (Hoyos Gómez 1979; Smart 1984, 1986; Hoyos Gómez and Herrero, 1989; Fernández- Gibert et al. 1992, 1994, 2000). 123 30 The present study documents a methodological approach useful for the geological and geomorphological characterization of these special environments and discusses the conditioning of karst development. Setting Torca Teyera is a large shaft, 738 m deep, located on the northern part of the Picos de Europa (Fig. 1), a mountain massif located in the Cantabrian Mountains of northern Spain. From the structural standpoint, Picos de Europa belongs to the Cantabrian Zone of the Variscides domain (Lotze 1945; Julivert et al. 1972; Alonso et al. 2009). The bedrock consists mainly of 1,200 m of carboniferous limestone affected by E–W to NW–SE and south-directed imbricate variscan system thrust (Fig. 2). The décollement level of the structures is above siliciclastic rocks from the Pisuerga-Carrión province (PérezEstaún et al. 1988; Marquı́nez 1989; Farias and Heredia 1994; Bahamonde et al. 2007; Merino-Tomé et al. 2009). During the Alpine orogeny, some of these thrusts were reactivated, causing the rotation of some thrust sheets Fig. 1 Situation map of the Picos de Europa massif. The locations of the cave of study (Torca Teyera, Fig. 5) are also shown 123 Carbonates Evaporites (2011) 26:29–40 and leading to the formation of the main relief (Alonso et al. 1996; Pulgar et al. 1999; Gallastegui Suárez 2000). Picos de Europa is characterized by a rough and calcareous relief with peaks exceeding 2,500 m above sea level (asl) and by the presence of narrow canyons, such as the Cares Gorge. Canyons up to 2,000-m deep evidence the important fluvial incision derived from uplifting. The karst forms dominate the landscape (Hoyos Gómez 1979; Smart 1984, 1986; Hoyos Gómez and Herrero 1989; Santos Alonso and Marquı́nez Garcı́a, 2005), although glacial and periglacial features are preserved (Alonso 1991; González Suárez and Alonso 1994; Gale and Hoare 1997; Alonso 1998; Jiménez-Sánchez and Farias Arquer 2002; González Trueba 2006, 2007; Moreno et al. 2009; Serrano Cañadas and González Trueba 2004). Moreover, nival, gravity and fluviotorrential processes also control the geomorphological evolution of the landscape. Torca Teyera shaft was discovered, explored and surveyed by the Groupe Spe´le´o du Doubs, the Socie´te´ Suisse de Spe´le´o-Section de Gene`ve, the Socie´te´ des Amateurs des Caverns and the Spe´le´o Club of Nyon between 1979 and 1982 (Borreguero 1986). During these explorations, Carbonates Evaporites (2011) 26:29–40 31 Fig. 2 Geological map of Picos de Europa (after Martı́nez Garcı́a and Rodrı́guez Fernández 1984; Marquı́nez 1989; Merino-Tomé et al. 2009). Location of Torca Teyera (Fig. 5) is shown Borreguero (1986) prepared the first karst research presenting the structural control and cave development. From 2007 to 2009, 2,700 m of new cave passages were discovered by the Asociación Deportiva GEMA. At the present, Torca Teyera has 4 km of known passages reaching a depth of 738 m. 123 32 Methodology The present methodology includes multidisciplinary observations to obtain both surficial data and data from the underground of the cave. The method is adapted to attain access to the shaft, which is difficult, and is based on speleological (Butcher 1950), geomorphological and structural geology techniques (Alonso et al. 1999; Jiménez-Sánchez et al. 2004, 2005, 2006). The method includes the definition of an area of 12 km 2 (Figs. 1, 2) including the cave and its surroundings: (1) the speleological survey at a 1:500 scale; (2) the geomorphological mapping of the cave at a scale of 1:500 and surrounding of the cavity at a 1:5,000 scale; and (3) the structural study that includes the geological and fracture mapping at a 1:5,000 scale, the cross section, rose diagram analyses and the definition of the joints families on stereographic projection. Cave survey and morphometric analyses The cave survey that corresponds to the Torca Teyera shaft is the cave projection in a horizontal plane. The survey was mapped using the speleological classical method at a 1:500 scale where successive stations were defined in the passages. Distances, orientation and dip data were measured between stations using a tape measure or laser, a ruler and a clinometer, respectively. The cavity survey was made by considering a reference level between 30 cm and 1.5 m above the cave floor. The collected data were managed using the VisualTopo.503 software program (David 2009) to define the survey line (a line connecting the stations). The survey is conducted over the survey line consulting the sketches made during the data collection phase. The VisualTopo.503 software produces a 3D model approximating the passages by an octagonal conduit, the axes of which are the height and width of the passages. The morphometric analyses consist of the representation of the survey data on stereographic projection, a common structural technique applied to compile figures on the direction and dip of the cave passages. The orientation and inclination data are represented on the plot per meter of surveyed cave. Afterward, a density analysis is made and the main groups of passages established according to this direction and dip. Geomorphological research The geomorphological research includes geomorphological mapping of both cave and cavity surroundings. Cave geomorphological mapping was carried out at a 1:500 scale, taking the cave survey as a topographical basis. Cavity features were inventoried and classified according to genetic, morphological and sedimentary criteria (Jiménez- 123 Carbonates Evaporites (2011) 26:29–40 Sánchez et al. 2006; Ford and Williams 2007). Sometimes, the geomorphological mapping and the survey were performed simultaneously. The limits of the different features were established and projected on the survey map. Some of the geomorphological elements located in the cave walls could not be represented by projecting them on the survey, since the survey had to be made at different heights above the ground. This problem was due to the difficulties in establishing objectively the boundary between the floor and the wall of the cave. Therefore, the geomorphological elements are shown schematically on the outside contour of the passages to minimize these problems. These elements were brought down on the walls along an axis located on the edge of the passages. A geomorphologic mapping of the cave surroundings was charted at a 1,500 scale using field observations and photo interpretation. This map covers a surface of 12 km2 and includes different landscape features that are classified according to genetic criteria (Martı́n-Serrano et al. 2004) in karstic, glacial, snow, periglacial, gravity and mixed forms. This map also includes the entrances of the caves and the projection of their passages that have been explored by the speleologists (Borreguero 1986; Carbajal Rodrı́guez and Saiz Barreda 2003); Carbajal et al. 2008; Ballesteros et al. 2009, 2010). Geological mapping and structural analyses The geological and fracture maps, covering a combined total surface of 12 km2 (Fig. 2), were produced at a 1:5,000 scale by means of field work and photo interpretation. Three geological cross sections were also prepared. The cavity study and others were projected over the maps and over the geological cross sections. Rose diagram from the data of the fracture map was prepared to compare with a rose diagram obtained from the orientation of the cave passages. Furthermore, 157 joint data measures (dip and direction) were taken on the surface (124) and in the shaft (33). These data were represented on stereographic projection and a density analysis was made. The analysis of densities allows the establishment of joint families in which the median plane is illustrated in the plot with the bedding. Afterward, the density plot of the orientation and dip of the passages were represented on the stereographic projection to compare the control of the joints and the bedding with the direction and inclination of the passages. Results and discussion The application of the methods described above obtained the following results. Carbonates Evaporites (2011) 26:29–40 Cave survey and morphometric analyses The cave survey and the 3D model are shown in Fig. 3. The cavity consists of three levels of galleries (horizontal passages) and several pits (vertical passages). The galleries represent 72% of the development of the cave and are narrow meanders up to 50 m that join as tributaries and converge to the NE. Thus, the cavity is a branchwork cave defined by Palmer (1991). The passages follow the NW–SE and NE–SW direction in the northern part of the cavity and the N–S and E–O trend in the southern area. The groups of passages according to their orientation and dip were determinate by the representation of the survey data on stereographic projection. This approximation is sufficient for passages, but not fully adequate for shafts because the survey depends on the track of the speleologists. Therefore, a part of the subvertical measures does not represent subvertical passages. This fact has been taken into account for the interpretation of data. Four groups of passages were established and the median value of the direction and dip are: (1) subvertical, (2) N10°W/ 20°NW, (3) N45°E/20°NE and (4) N125°E/0°. Cave geomorphological mapping A selected portion of the geomorphological map of the southern sector of the shaft is illustrated in Fig. 4. The legend of the map is divided into three parts: issues related to (1) the survey, (2) geomorphological features and (3) geological aspects. The first group includes morphometric data related to the passages: contour (using the upper and lower contour when there is an overlap), scarps and pits, presence of rivers or lakes, the slope of the ground and the value of altitude and depth from the cave entrance at several points. The position of possible continuations of 33 cave passages is also shown. The group of geomorphological features includes (1) speleothems, (2) fluviokarst and (3) gravity forms. Speleothems are classified into dripstones, flowstones, and mixed and other forms. Dripstone forms include stalagmites, stalactites and columns. The flowstones present as cascades and laminar forms. The mixed forms include stalagmite masses that originated by drip and flow of water (Fig. 4a). Other peculiar forms such as pool deposits or collaroids forms are noted. The fluviokarst forms are classified as erosive forms or sedimentation forms. The erosive forms include scallops, roof pendants, corrosion notches, solution runnels, potholes and relict channels. The erosive forms are mostly located in active and canyon shaped passages, shafts and at higher levels, such as relict forms (Fig. 4b). The fluvial deposits are divided into deposits of the active stream channel and the fluvial terrace (Fig. 4c). These deposits have been classified according to the grain size as pebbles, sand and pebbles, sand, and clay and mud. Finally, the gravity forms are debris deposits, fallen boulders and single pebbles and gravel that have been shed by rockfall processes (Varnes 1978). The breakdown deposits cover or are covered by other fluvial and precipitation deposits. The geomorphological map also includes other remarkable geological aspects: quartz, galena and malachite mineralizations, altered substrate, structural data and volcanic rocks. Geomorphological mapping of the cave surrounding Torca Teyera is located under a free and half-exposed karren, dominated by karstic, glacial and nival activity. The geomorphologic map of the cave surroundings is shown in Fig. 5a. The distribution of the geomorphological features is uneven. The deposits represent 27.3% of the area of study and are mainly situated in the valleys. The erosive Fig. 3 a The cave survey and b the 3D model of Torca Teyera. The study location described in Fig. 4 is shown 123 34 Fig. 4 Details from a selected portion of the cave geomorphology map, its legend and pictures of different passages. Inset a represents a gallery shaped like a canyon, b a stalagmite mass and c terrace deposits formed by levels of mud and sand 123 Carbonates Evaporites (2011) 26:29–40 Carbonates Evaporites (2011) 26:29–40 35 Fig. 5 a Geomorphological map, b geological map and c fracture map of Torca Teyera area. The shaft is projected on maps and cross section of Fig. 6a. The data of the caves are from Borreguero (1986), Carbajal Rodrı́guez and Saiz Barreda (2003), Carbajal et al. (2008), Ballesteros et al. (2009, 2010) forms have been identified at the hillside and the peaks. The closed depressions occupy 5.0% and are developed on slopes up to 40°. Karstic, snow, glacial, periglacial and gravity forms were mapped. The former includes karstic deposits, dolines, glaciokarst depressions, caves and cave passages projections. The karst deposits are situated in the areas without slope and are mainly formed by mud that comes from the dissolution of the limestone. The sinkholes are depressions of 5- to 20-m wide and 1- to 5-m depth and associated with gravitational deposits. The dolines are closed and dominated by breakdown processes; thus, they are mostly collapsed sinkholes. The glaciokarst depressions are closed hollows of 600-m wide and 70-m depth located 123 36 at the valley bottom. The depressions originated from glacial, karstical and snow activity (Smart 1986; Alonso 1998). The entrances of caves are mainly situated in sinkholes or glaciokarst depression and represent cave passages truncated by erosion. Torca Teyera is located under a free karren with a lot of closed depressions, gravity deposits and some glacier cirques. The nivation forms include one moraine and some cirques originated by snow activity. The nivation cirques are observed in some walls of dolines and in scarps oriented to the NE or SW. The snow moraine is located under one nival cirque. Glacial forms cover till deposits, horns, arête, cirques and moraines. A glacial valley is observed in the NW of Torca Teyera shaft. Till deposits, mainly formed by limestone pebbles and boulders, sand and mud, are found in this valley. In some cases, boulders of limestone in certain facies are found on the bedrock of limestone with other facies. The till is often presented in moraines of 1- to 6-m wide and 50- to 60-m long. The horns are degraded and are situated in the peaks where some arêtes converge. The cirques are greatly degraded by karstification and their size is between 100 and 300 m. The only periglacial evidence is a rock glacier shown on the eastern part of the map. The deposit is formed by limestone gravels and boulders, and presents some transverse and longitudinal ridges and furrows. Gravitational forms include debris fall, talus deposits and rock avalanches and are situated under scarps and in sinkholes. The debris fall generally consists of angular pebbles and gravels of limestone. The talus deposits are mostly formed of limestone pebbles and gravel, sand and mud; these are usually vegetated. The rock avalanches are formed from disorganization of angular boulders and pebbles of limestone in the NE of the map. Some geomorphological features (till, closed depressions, cirques, arêtes, gravity deposits and the rock glacier) are mainly orientated following the NW–SE and NE–SW directions. This trend represents the orientation of the bedding, thrusts and the main faults. Geologic mapping and structural research The cavity surrounding of the geological map was formed by both the cave development and the tectonics (Fig. 5b). The surroundings of Torca Teyera were formed by 1,000 m of limestone of the Valdeteja and Picos de Europa Formations stacked vertically. The limestone is divided into three strata domain as defined by Bahamonde et al. (2007): (1) toe of slope and basin facies, (2) slopes facies and (3) platform top facies. The toe of slope and basin facies include well-stratified coarse-grained beds formed by breccias, bioclastic pack to grainstone limestone, chert and shales. The slopes facies consist of massive limestone, 123 Carbonates Evaporites (2011) 26:29–40 breccias and boundstone with botryoidal cement fans. The platform top facies include stratification rocks, mainly formed by skeletal pack, to grainstone limestone and pink fossil-rich limestone. Moreover, an andesitic dyke is recognized in the western middle part of the geological map. The cavity studied is developed on limestone affected by an NW–SE trending, subvertical and SW-directed thrusts and other faults, the direction of which is SE–NW, SW–NE or N–S (Figs. 5b, 6a). Two sequences of thrusts have been recognized based on their geometric relationships. The first sequence dips 50–70 NE and is interrupted by an out-ofsequence anomaly, which dips by 80–90° NE. Figure 5a shows 2,367 fractures in the study area; these are represented in a rose diagram (Fig. 6b). The plot shows three groups of fractures with directions: (1) NE–SW, (2) N–S and (3) NW–SE. The first group represents 45% of the total fractures and the second group represents another 17%. The direction of the cave passages are analyzed by another rose diagram (Fig. 6c). On comparing both (fracture and cave direction) rose diagrams, the first structural control of the orientation can be semiqualitatively established. The dispersion of values of the shaft orientation is greater than the fracture data, although the NW–SE direction of the cave is noted. The group considered as Fracture 1 is the most abundant collection, but its influence on the cavity development is less than the group considered as Fracture 2. This method does not consider the influence of the bedding and the intersection between discontinuities; consequently, it is only a first approximation to the structural factor. Seven families of joints have been established on the stereographic projection. The average plane of each family is represented in Fig. 7a. The families J2, J3 and J5 correspond to the fractures already recognized in a previous work (Borreguero 1986). The density plot of the orientation and dip of the passages has been shown in gray scale on the stereographic projection (Fig. 7a) to compare the main orientations and dips of the joints, the bedding and the cave passages. Figure 7a highlights that subvertical cave passages (Group 1 of galleries) are conditioned by the joint families J1, J3, J4 and J6, as well as their intersections. The galleries belonging to Group 2 (N10°W/20°N) are mainly controlled by the intersection between the families J5 and J6. The passages dipping down 20° to the NE (Group 3) are ruled by the intersection between family J1, J2, J5 and J7. The latter group of passages consists of horizontal galleries in the direction N125°E and follows the bedding (Fig. 7b). Conclusions The use of a multidisciplinary methodology including the speleological cave survey, geomorphological mapping and Carbonates Evaporites (2011) 26:29–40 37 Fig. 6 a One of the three cross sections prepared, the position of which is shown in Fig. 5b. b Rose diagram of the fracture direction and c of the orientation of Torca Teyera passages structural techniques is adequate to develop reliable geomorphological assumptions for a cave with difficult access. Also, this multidisciplinary methodology allows the definition of factors controlling karst development, especially the quantitative evaluation of the structural influence on endokarst. The speleological cave survey is a graphical document that is useful to plot geological and geomorphological information. However, its development is complex due to the adversity of the environment and the subjective criteria of the researcher who has to include different elements located at inconstant heights above the floor. The 3D model approximates the geometry of the endokarst system as a whole. In the scale of the passage, the approximation is not correct because the irregularities of the passage walls are not seen. When a section of a gallery or a shaft is not subcircular, the model does not properly represent the shape of the passage because the software uses an octagonal section to approximate the shape of the section. Where the galleries display a canyon morphology, modeling results are not accurate because, in the model, the value of passage widths decreases with the distance to the floor and the roof. After modeling, further analyzing the cave survey data with stereographic projection is useful to quantitatively classify the cavity passage according to the direction and dip. Cave geomorphologic mapping indicates the different forms and their spatial distribution. The map informs about the genetic processes and their spatial and temporal relationships. These aspects are the base of the speleogenetical model of the shaft. Some limitations of the method are conditioned by the precision of the cave survey. In the wall of the passages, several interesting forms are present; nevertheless, these forms cannot be represented on the cave survey because the plot is only a horizontal projection. If the forms on the walls are projected over the survey, all of them are situated in the same place. The imprecision of information on walls can be solved by projecting down the forms on the cave limits. 123 38 Carbonates Evaporites (2011) 26:29–40 Fig. 7 a Stereographic projection of the orientation and dip of the passages showing the position of the planes that represent the main families of joints and the bedding; b a cave passage that has been controlled by the bending; and c the structural control of each group of galleries The geological and fracture mapping of the cave surrounding and the cross section with the shaft projection determine the qualitative structural control of the cave. The comparison between the fracture direction and the orientation of the cavity passage allows a first hypothesis about the structural control. This comparison does not consider the bending, dip or the hypothetical intersection between the discontinuities. These limitations can be resolved using stereographic projection, where the dip and the direction of joints, bedding and cave passages are represented. This plot that includes survey data together with structural data evaluates quantitatively the relationship between the structure and the endokarst. Acknowledgments This research has been funded through the CONTRACT project (CN-06-177) provided by ASTURIAS GOVERNMENT-OVIEDO UNIVERSITY, CALIBRE project (CAVECAL) (CGL2006-13327-C04/CLI) provided by Ministerio de Educación y Cultura and GRACCIE project (CONSOLIDER PROGRAM) (CSD2007-00067) provided by Centro de Investigación Cientı́fica y Tecnológica. We acknowledge Dr. Juan Bahamonde, Dr. Óscar Merino, Gemma Sendra, Irene de Felipe, Asociación Deportiva Gema, Grupo Espeleológico Polifemo and GES Montañeiros Celtas for their help. 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